Air-conditioning control system for minimizing offset and zone temperature oscillations

ABSTRACT

An air-conditioning system for a zone is disclosed which includes air-heating and air-cooling apparatus which are cycled for heating or cooling air moving to the zone through a duct. Cycling of the apparatus is governed by a temperature control system including a zone air temperature sensor and a duct air temperature sensor. The duct sensor has a relatively large authority in affecting control of the air-conditioning apparatus when the heating or cooling load on the zone is light so that variations in the duct air temperatures alone can effect cycling of the apparatus without any appreciable variation in the zone air temperature from a set point temperature. The authority of the duct sensor diminishes substantially when the zone temperature is relatively remote from the set point so that the conditioning apparatus is not cycled according to the duct air temperature.

United States Patent Lorenz 1 Feb. 29, 1972 OSCILLATIONS [72] Inventor:Jerome L. Lorenz, Columbus, Ohio [73] Assignee: Ranco Incorporated,Columbus, Ohio [22] Filed: Sept. 23, 1970 [21] Appl. No.: 74,305

[52] US. Cl ..l65/27, 165/14, 165/22, 62/ 160 [51 Int. Cl ..F25b 29/00[58] Field of Search ..165/27, 22, 14; 62/160 [56] References CitedUNITED STATES PATENTS 3,512,579 5/1970 Muskorac ...l65l26 3,567,11512/1970 Nelson ..l65/22 Primary Examiner-William .1 Wye Attorney-Watts,Hoffmann, Fisher & Heinke [57] ABSTRACT An air-conditioning system for azone is disclosed which includes air-heating and air-cooling apparatuswhich are cycled for heating or cooling air moving to the zone through aduct. Cycling of the apparatus is governed by a temperature controlsystem including a zone air temperature sensor and a duct airtemperature sensor. The duct sensor has a relatively large authority inaffecting control of the air-conditioning apparatus when the heating orcooling load on the zone is light so that variations in the duct airtemperatures alone can effect cycling of the apparatus without anyappreciable variation in the zone air temperature from a set pointtemperature. The authority of the duct sensor diminishes substantiallywhen the zone temperature is relatively remote from the set point sothat the conditioning apparatus is not cycled according to the duct air2,155,256 4/1939 Crogo ..165/27 temperamm 2,218,468 lO/l940 Haines.......l65/22 2,372,839 4/1945 McGrath ...l65/22 9Claims,4DrawingFiguresl-EF SIGNAL RELAY 7 AMPLIFIER 32 l 5- -c/Rcu/my HEAT/N6 I P RELAY r15 i10' J J I za n- J l I i 16' K I i 18 g s f 2 24 I 42 Q AIR-CONDITIONINGCONTROL SYSTEM FOR MINTMIZING OFFSET AND ZONE TEMPERATURE OSCILLATIONSBACKGROUND OF THE INVENTION Field of the Invention The present inventionrelates to control systems for air-conditioning systems and moreparticularly to control systems governing operation of relatively largecapacity air-conditioning systems for heating or cooling air in a zonesuch as a building or a room in a building.

The Prior Art Air-conditioning systems in which heated or cooled air isducted into a zone have been in common use. in some systems, the cyclicoperation of the air heating or air-cooling units was governed bycontrol systems having sensors responsive to duct air temperature andzone air temperature. The temperature sensors produced mechanical orelectrical signals to control on-off operation of the air-conditioningequipment.

In such systems the duct air temperature has been said to lead the zoneair temperature, i.e., the duct air temperature was elevated or reducedbefore the temperature of the air in the zone was elevated or reducedrespectively. The duct air sensor thus provided a leading control signalwhich was used to modify the affect of the zone sensor signal. Since theduct air temperature fluctuated over a wide range compared to the zoneair temperature, the control system was arranged so that the duct airsensor had a significantly smaller authority in operating the equipmentthan did the zone sensor. For example, when the duct sensor detected a1' F. duct air temperature change, the duct sensor signal was alteredonly slightly and therefore its authority over operation of theair-conditioning equipment was small. On the other hand when the zoneair sensor detected a 1 F. temperature change, it produced a relativelylarge responsive signal having a relatively great authority overoperation of the air-conditioning equipment.

it was recognized that large capacity air-conditioning equip ment tendedto cause unduly large oscillations in the zone air temperature when thezone was lightly loaded, i.e., when the zone temperature varied butslightly from the set point temperature. For this reason, the authorityratio between the duct and zone sensor signals has been maintainedrelatively low. For example, it was common for systems to employauthority ratios of about 20:1 where oscillations at light cooling loadswere to be avoided. Thus, introduction of excessively highorlow-temperature air into the lightly loaded zone was avoided and thezone air temperature did not oscillate unduly when the load was light.

The requirement that low-authority ratios be used to prevent oscillationat light loads, however, resulted in substantial offsets, or thermostatdroop, when the zone was heavily loaded, i.e., when the temperature ofthe zone air varied widely from the set point temperature. Offset, ordroop, usually occurred when the zone was heavily loaded and thethermostatic controls for the system prevented the set point temperaturefrom being maintained. If, for example, the set point (and desired) zonetemperature was 75", offset or droop resulted in stabilization of thezone temperature at below 70 when the heating load on the zone was high.This was due to the relatively high authority duct sensors which cycledthe heating equipment while the zone temperature remained below the setpoint. In order to increase the zone temperature to 75, the set pointtemperature had to be increased to 85, for example.

Offset also occurred where the zone was being cooled and the coolingload on the zone was high. In the case of cooling, the zone temperatureusually stabilized well above the set point temperature.

The occurrence of offset, whether during heating or cooling, requiredthat the zone set point temperature be frequently readjusted in order toproduce a desired zone temperature level.

The prior art systems have generally employed a sensor authority ratiowhich was intermediate the optimum ratio for avoiding undue zonetemperature oscillations at light zone loadings and an optimum ratio foravoiding excessive droop.

SUMMARY OF THE INVENTION The present invention provides a new andimproved control for an air-conditioning system wherein the zonetemperature oscillation is minimized when the load on the zone is light,yet offset, or thermostat droop, is minimized when the load on the zoneis large.

In a preferred form of the invention, an air-conditioning systemprovides heated or cooled air to a zone via suitable ducting. The zoneair temperature is determined by a control system which governsoperation of the air-conditioning system.

The control system includes a zone air temperature sensor and a duct airtemperature sensor. The zone air sensor produces a signal proportionalto the difference between the zone air temperature and a preselected setpoint temperature. The duct sensor produces a signal proportional to thedifference between the duct air temperature and the set pointtemperature. These signals are combined and processed by the controlsystem which produces a command signal for controlling theair-conditioning system. Thus the operation of the air-conditioningsystem is governed by a command signal which is proportional to thecombined signals from the duct and zone sensors.

The duct sensor and its associated circuitry generates a duct sensorsignal which varies as a nonlinear function of sensed duct temperature.When the zone is relatively lightly loaded, i.e., when the zonetemperature is close to the set point temperature, the airconditioningequipment is controlled by the combined zone sensor and duct sensorsignals which have an authority ratio of approximately 20:1. When thesensed zone air temperature varies from the set point temperatureheating or cooling, as the case may be, is called for. Conditioned airis supplied to the zone through the duct and the temperature of theconditioned air discharged to the zone is sensed by the duct sensor.

Because the authority ratio between the zone sensor signal and the ductsensor signal is relatively small for light heating or cooling loads,the change in duct air temperature detected by the duct sensor producesa significantly changed duct sensor signal. This signal substantiallyeffects the command signal tending to modulate or cycle theair-conditioning equipment. This occurs without requiring anysignificant change in the zone temperature signal. The zone temperatureis thus prevented from oscillating substantially when the zone islightly loaded and large capacity heating and/or cooling units areassociated with it.

When the zone is relatively heavily loaded, the zone temperature tendsto be substantially different from the set point temperature of thethermostat due to heat transfer to or from the zone. When this occurs,the new control system enables the air-conditioning equipment to supplysufficient heated or chilled air to the zone to return the zone airtemperature to the set point level without requiring the zone thermostatto be reset in order to do so.

The new system provides a duct sensor signal which exhibits a small rateof change of magnitude with respect to sensed duct air temperaturechange when the zone is heavily loaded. In effect, the control systembecomes insensitive to large duct air temperature deflections when thezone loads are large. This enables relatively hot, or relatively coldduct air to be in troduced into the zone without cycling or modulatingthe airconditioning equipment. Hence the air-conditioning equipment isprimarily responsive to the sensed zone air temperature since thecommand signal more closely reflects sensed zone air temperature thanduct air temperature.

Where the heating or cooling load on the zone is large and ture, theauthority ratio between the zone sensor signal and the duct sensorsignal can be on the order of 40:1 or 50:1 or higher.

The new system is particularly effective for use in air-conditioningsystems which employ electrical resistance heating elements to heat theduct air entering the zone. These heating elements have relatively smallthermal inertias, so that heating the air in a given zone can beaccurately controlled by the use of a sensor and control arrangementcapable of rapidly cycling the heaters. Furthermore, then the heatingloads on the zone are heavy, the heating elements can be operated for asignificant period of time without frequent cycling caused by a duct andzone sensor arrangement in which a low-authority ratio between the zonesensor signal and the duct sensor signal is maintained.

A principal object of the present invention is the provision of a newand improved control system for an air conditioned zone wherein theauthority ratio between a zone air temperature signal and a duct airtemperature signal is relatively low when the heating or cooling load onthe zone is light and which authority ratio increases markedly when theheating or cooling load on the zone increases thereby enabling a largecapacity air-conditioning system to control a lightly loaded zone withminimal air temperature fluctuations in the zone, yet which enables thezone to be heated or cooled under large loads with minimal offset orthermostat droop.

Other objects and advantages of the present invention become apparentfrom the following detailed description made with reference to theaccompanying drawings which form a part of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of anair-conditioning system embodying the present invention;

FIG. 2 is a diagrammatical view of circuitry forming a part of thesystem of FIG. 1; and

FIGS. 3a and 3b are graphic representations of signals produced by azone temperature sensor and a duct temperature sensor, respectively,utilized in the system of FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT An air-conditioningsystem is illustrated in FIG. 1. The system 10 includes an airconditioned zone 12 which may be a room in a building and which zone isprovided with a rooftop air-cooling apparatus 14 and a rooftopair-heating apparatus 15. Air is circulated through the zone 12 viaducting 16 which includes an air blower unit 18. The zone airtemperature is modulated by a control system 20 which governs operationof the air-cooling apparatus 14 or the air-heating apparatus dependingon the nature of the heating or cooling load on the zone.

The air-cooling apparatus 14 includes a refrigeration unit having anair-cooling heat exchanger 24. The refrigeration unit can be acompressor-condenser-evaporator-type mechanical unit with the heatexchanger 24 being formed by the evaporator of the unit. The heatexchanger 24 is disposed in the ducting 16 so that duct air is directedacross the heat exchanger 24 for cooling.

The air-heating apparatus 15 includes an air-heating heat exchanger 28disposed in the ducting 16 so that the air circulating into the zone 12is directed across it for heating. The heat exchanger 28 is preferably aseries of banks of electrical resistance heated coils. These heaters maybe of any suitable type and configuration and may be commerciallyavailable types, accordingly, the heating apparatus 15 and refrigerationapparatus 14 are not described in further detail.

The control system operates the heating and cooling units in response tosensed duct air temperature and sensed zone air temperature. The system20 operates the rooftop aircooling apparatus 14 by a cooling relay unit30. The rooftop air-heating apparatus 15 is operated by a heating relayunit 32. The relay units 30, 32 may be of any suitable construction andtherefore are schematically shown.

The relay units 30, 32 are connected to the output circuit of adifferential amplifier 36. The amplifier 36 provides a command signal tooperate the relay units and is connected across positive, negative andneutral, or reference, terminals of a DC power supply. When theamplifier output voltage levels are positive with respect to thereference terminal, the cooling relay is operable. When the outputvoltage from the amplifier 36 is negative with respect to the referencevoltage, the heating relay is operable.

The input signal for the amplifier 36 is provided by a zone sensor 40and a duct sensor unit 42. The duct sensor produces an electrical signalwhich is a function of the difference between a sensed duct airtemperature and a temperature set point for the zone. This signal is fedto the input of the amplifier 36 through a lead 44. The zone sensordetects air temperatures in the zone and produces an electrical signalwhich is fed to the amplifier through a lead 46. The signal produced bythe zone sensor is a function of the difference between a detected zoneair temperature and a zone temperature set point.

The zone and duct sensor signals are combined algebraically to provide acomposite signal to the amplifier 36. The composite signal is a functionof the zone air temperature and duct air temperature. In the illustratedembodiment, the leads 44, 46 are both connected to an amplifier inputlead 48. The amplifier 36 is a linear amplifier so that the commandsignal at the amplifier output is proportional to the composite inputsignal.

The amplifier input signal is varied more by the zone sensor than by theduct sensor. That is to say, the change in input signal level producedby the detection of a 1 F. temperature increase by the zone sensor issubstantially larger than the change in input signal level produced by adetected 1 F. air temperature increase by the duct sensor.

In order to prevent oscillation of zone air temperature by ahigh-capacity air-conditioning system when the zone is lightly loaded,the authority ratio between the zone air signal and the duct air signalis relatively low. For example, the ratio between the change inamplifierinput signal produced by one degree temperature change detectedby the zone sensor and the change in input signal produced by a 1temperature change sensed at the duct sensor is approximately 20: 1.

At this authority ratio, the effect of the duct sensor on the amplifieroutput signal is relatively large and the heating or cooling apparatusis cycled in response to fluctuations in the temperature of the duct airrather than fluctuations in the zone air temperature. Thus the ductsensor can be said to provide a leading signal which causesstabilization of the zone temperature without substantial overshootingresulting from the thermal inertia of the zone and its contents.

The zone air temperature is thus stabilized at or about the set pointtemperature without marked oscillations in the zone air temperaturesince heating or cooling of the zone occurs cyclically and as determinedby the temperature of the duct arr.

Where the heating or cooling load on the zone is large, the sensed zonetemperature is substantially different from the set point temperature.In these conditions it is essential that the authority ratio berelatively high so that adequate quantities of hot or cold air can beintroduced into the zone through the duct. Put another way, whentheheating or cooling load on a given zone is large, the effect of the ductsensor signal on the command signal should be minimized.

If the authority ratio between the zone air sensor signal and the ductorsensor signal were relatively low during periods of large loads on thezone, the air-conditioning equipment would be cycled in response tochanges in duct air temperature. This would occur essentiallyindependently of the sensed zone air temperature and theair-conditioning equipment will continue to cycle without balancing theload on the zone, i.e., the sensed zone temperature remainssubstantially different from the set point temperature. An occupant ofthe zone may be required to change the zone set point temperature to anartificially high or low level in order to maintain a desiredtemperature in the zone. This phenomenon is known as temperature offsetor thermostat droop.

According to the present invention, the authority ratio between the zonesensor and the duct sensor varies depending on the difference betweenthe zone temperature and the set point temperature. The zone and ductsensors have a relatively low-authority ratio when the heating orcooling load on the zone is small and a relatively high-authority ratiowhen the heating or cooling load on the zone is high.

FIG. 2 shows preferred electrical circuitry forming the zone and ductsensor circuits. The zone sensor 40 is formed by a thermally responsivevoltage divider arrangement connected across the positive and negativepower supply terminals. The voltage divider arrangement includes a zonetemperaturesensing thermistor 50, a potentiometer 52 and a fixedresistor 54 connected in a series circuit across the power supplyterminals. A linearizing resistor 56 is connected in parallel with thethermistor S0.

The output of the voltage divider circuit is transmitted to theamplifier 36 via a junction 58, the lead 46 and the amplifier input lead48. The zone sensor circuitry provides a substantially linear voltageresponse to temperature changes throughout the range of predictable zonetemperatures. The slider of the potentiometer 52 is mechanically linkedto a temperature setting knob or dial of the zone sensor unit 40 so thatmanual setting of the resistance of the potentiometer 52 determines thezone set point temperature. The knob is not shown.

The circuitry for the duct sensor 42 includes a thermistor 60 connectedin series with a fixed resistor 62 via an output junction 64. A resistor66 connected in parallel with the thermistor 60 has a resistance valueselected to linearize the resistance of the thermistor 60 over apredetermined temperature range. The output voltage from the duct sensoris transmitted to the amplifier input through a resistor 70 and apotentiometer 72.

The potentiometer 72 is adjustable to control the authority ratiobetween the duct sensor and room sensor circuitry at a given level. Whenthe thermistor 60 and the thermistor 50 both sense air at 75 F. thepotentiometer is adjusted so that the signal produced by the zone sensorcircuitry is approximately twenty times the magnitude of the signalproduced by the duct sensor circuitry. This adjustment is preferablymade during installation of the system.

The output signal from the amplifier 36 attributable solely to the zonesensor circuitry is substantially a linear function of sensed zone airtemperature extending throughout the range of amplifier output voltages.FIG. 3a shows the amplifier output voltage curve due solely to thesensed zone air temperature. In a preferred embodiment of the invention,the amplifier produces a 100 percent positive output voltage at a zonetemperature 1V2" above the set point level and a negative 100 percentamplifier output voltage when the zone temperature is about 1 5 belowthe set point temperature.

The duct sensor circuitry illustrated in FIG. 2 enables the authorityratio between the zone sensor and the duct sensor to increase markedlywhen the zone temperature differs substantially from the set pointtemperature, i.e., when the load on the zone is great. This function ofthe duct sensor circuit is illustrated in FIG. 3b wherein the amplifieroutput voltage is shown solely as a function of sensed duct airtemperature. The plot of the output voltage attributable to signals fromthe duct sensor is a nonlinear curve having a linear, or substantiallylinear, portion extending through the set point. This curve exhibits arelatively small rate of change of voltage with respect to sensed ductair temperature at duct air temperatures which are substantially higheror substantially lower than the set point temperature.

The duct sensor circuitry thus prevents thermostat droop as well as zonetemperature oscillations at light loads. As an example of this function,assume that a light cooling load is applied to a zone cooled by ahigh-capacity cooling system. The light cooling load can result in thezone air temperature rising to l F. above the set point temperature. Thezone sensor circuitry detects this zone air temperature and consequentlya positive amplifier output or command voltage of about 30 percent ofpeak is generated.

The cooling apparatus is operated in response to this amplifier outputvoltage resulting in a forced flow of chilled duct air being circulatedin the zone. The chilled air in the duct is substantially cooler thanthe set point temperature so that the duct air sensor produces acorresponding signal. It, for example, the duct air temperature isreduced to 10 F. below the set point temperature the signal produced bythe sensed duct air temperature reduces the amplifier output voltagenegatively towards the reference level. The reduced command voltagelevel turns off the cooling equipment without substantially depressingthe zone temperature.

The duct air temperature then rises toward the set point temperature andthe cooling apparatus is cycled on again, unless the zone airtemperature has been reduced to nearly the set point level. Cycling ofthe refrigeration apparatus continues under the control of the duct airsensor until the zone temperature is sufficiently close to the set pointtemperature that the cooling equipment can no longer be cycled.

On the other hand, if the cooling load on the zone is extremely high thesensed zone temperature is likely to be at least 3 F. above the setpoint temperature. This produces an amplifier output voltage of percentof the peak value. When this occurs, the cooling apparatus is operatedby the amplifier and cooling relay to supply chilled air to the zone.

As chilled air is supplied to the zone, the duct sensor produces anamplifier input signal which is of a negative sense and which tends todiminish the input signal to the amplifier. As the duct air temperaturecontinues to decrease, the duct sensor begins to respond nonlinearly tochanges in the sensed duct air temperature. When this occurs the rate ofchange of the duct temperature signal with respect to sensed duct airtemperature is reduced to a relatively low level. The effect of the ductair sensor on the amplifier output voltage is thus reduced as the ductair temperature decreases and the zone air temperature remains highrelative to the set point temperature. The reduced sensitivity of theduct sensor to duct air temperatures remote from the set pointtemperature raises the authority ratio of the zone sensor to the ductair sensor markedly when the load on the zone increases. As a result therefrigeration unit can no longer be cycled in response to sensed ductair temperature as the signal produced by the duct air sensor is notsufficient to overcome the effect of the signal produced by the zonesensor. Accordingly, the unit runs continuously until the zonetemperature has been reduced to a level at which the combined duct airtemperature signal and zone air temperature signal cause cycling ormodulation of the refrigeration unit.

The variable authority ratio is also effective when the zone is heated.When large differences between the set point temperature and the zonetemperature exist the authority ratio is increased significantly. Theduct sensor tends to cycle the heating apparatus less frequently,enables higher duct air temperatures to exist and therefore permits thezone air temperature to approach the set point temperature ratherclosely.

From the foregoing description it can now be seen that a new andimproved air-conditioning system for a zone has been provided in whichthe authority ratio between a zone sensor unit and a duct sensor unit isvariable to eliminate temperature oscillation in the zone at light loadsand to prevent offset, or thermostat droop, when the loads on the zoneare high.

While only a single embodiment of the invention has been illustrated anddescribed herein in considerable detail the present invention is not tobe considered limited to the precise construction shown. One electricalsystem for producing a variable authority ratio is disclosed; however,others could be devised. For example, a mechanical thermostatic element,such as a bellows, could be connected to a variable impedance element toproduce a nonlinear duct temperature signal for producing the variableauthority ratio. It is intended to cover hereby all adaptations,modifications and uses of the invention which come within the scope ofthe appended claims.

What is claimed is:

. 1. in a system for air conditioning a zone having air heating meansfor heating air circulating to the zone and air cooling means forcooling air circulating to said zone, a control system governingoperation of said heating and cooling means in response to sensedtemperature comprising:

a. a first air temperature sensor unit for sensing variations in zoneair temperature relative to a set point temperature and producing asignal which is a function of said sensed variations; a second airtemperature sensor unit for sensing the temperature of air dischargedinto said zone and producing a second signal which varies as a functionof the temperature of the air discharged into said zone;

0. control means for said heating and cooling means, said control meansconnected to said first and second sensor units whereby said first andsecond signals are combined to govern operation of said control meansaccording to the air temperatures sensed by said sensor units;

(1. said second sensor unit including temperature responsive circuitryhaving elements for producing a relatively large rate of change ofmagnitude of said second signal with respect to sensed temperaturechange when said discharge temperatures are close to the zone set pointtemperature and a relatively small rate of change of magnitude withrespect to sensed temperature change when said discharge airtemperatures are remote from said set point temperature.

2. A system as claimed in claim 1 wherein the ratio of authority of saidfirst sensor signal to the authority of said second sensor signalincreases as the heating or cooling load on said zone increases.

3. A system as claimed in claim 2 wherein said authority ratio is about:1 when the heating or cooling load on said zone is minimal andincreases to at least about 40:1 when the heating or cooling load onsaid zone increases.

4. A system as claimed in claim 1 wherein said first sensor unitcomprises an electrical resistance element having a resistance whichvaries according to the temperature of said element, said elementconnected in sensor circuitry which produces a substantially linearsignal in response to zone air temperature changes within apredetermined range detected at said resistance element.

5. A system as claimed in claim 4 wherein said circuit elements of saidsecond sensor unit include a second temperature responsive resistanceelement characterized by having a resistance which varies in response todischarge air temperature changes, said second resistance elementassociated with circuitry for producing a substantially linear signal inresponse to discharge air temperature changes within a predeterminedrange extending above and below the set point temperature and nonlinearsignals when said discharge air temperatures are beyond said range.

6. In an air-conditioning system for comfort conditioning air circulatedto a zone, a control system comprising:

a. control means governing operation of said system in response tosensed air temperatures; b. air temperature sensor means cooperativelyrelated to said control means; c. said sensor means comprising:

1. a zone air sensor device producing a zone air temperature signalwhich is a function of the difference between sensed zone airtemperature and a set point temperature;

2. a discharge air sensor device producing a temperature signal which isa function of the difference between the temperature of air dischargedto said zone and said set point temperature;

3. structure for combining said zone air temperature signal with saiddischarge air temperature signal whereby said control means responds tosaid combined signals; and

d. said discharge air sensor device including means providing for afirst relatively large authority of said discharge air temperaturesignal when said zone temperature is su stantially at said set pointtemperature and a second smaller authority when said zone temperature issubstantially different from said set point temperature, said airconditioning system controlled primarily by said discharge airtemperature signal when said authority is relatively large andcontrolled primarily by said zone air temperature signal when saidauthority is smaller.

7. A system as claimed in claim 6 wherein the ratio of the authority ofsaid zone air temperature signal to said discharge air temperaturesignal is about 20:1 when said zone air temperature is at said set pointtemperature, said authority ratio increasing to at least 40:1 as saidzone air temperature varies from said set point temperature.

8. In a control system for an air conditioned zone:

a. air-conditioning apparatus;

b. control means for said air-conditioning apparatus;

c. a zone air temperature sensor;

d. a discharge air temperature sensor;

e. means for establishing a set point temperature;

t. one of said sensors including means for changing the authoritythereof as the zone air temperature varies from said set pointtemperature whereby the ratio of the authority of said zone sensor tosaid discharge sensor varies from about 20:] when said set pointtemperature and zone temperature are substantially the same to at least40:1 when said zone temperature is substantially different from said setpoint temperature.

9. A system as claimed in claim 8 wherein the authority of saiddischarge sensor is reduced as the difference between said zone airtemperature and said set point temperature increases.

1. In a system for air conditioning a zone having air heating means forheating air circulating to the zone and air cooling means for coolingair circulating to said zone, a control system governing operation ofsaid heating and cooling means in response to sensed temperaturecomprising: a. a first air temperature sensor unit for sensingvariations in zone air temperature relative to a set point temperatureand producing a signal which is a function of said sensed variations; b.a second air temperature sensor unit for sensing the temperature of airdischarged into said zone and producing a second signal which varies asa function of the temperature of the air discharged into said zone; c.control means for said heating and cooling means, said control meansconnected to said first and second sensor units whereby said first andsecond signals are combined to govern operation of said control meansaccording to the air temperatures sensed by said sensor units; d. saidsecond sensor unit including temperature responsive circuitry havingelements for producing a relatively large rate of change of magnitude ofsaid second signal with respect to sensed temperature change when saiddischarge temperatures are close to the zone set point temperature and arelatively small rate of change of magnitude with respect to sensedtemperature change when said discharge air temperatures arE remote fromsaid set point temperature.
 2. A system as claimed in claim 1 whereinthe ratio of authority of said first sensor signal to the authority ofsaid second sensor signal increases as the heating or cooling load onsaid zone increases.
 2. a discharge air sensor device producing atemperature signal which is a function of the difference between thetemperature of air discharged to said zone and said set pointtemperature;
 3. structure for combining said zone air temperature signalwith said discharge air temperature signal whereby said control meansresponds to said combined signals; and d. said discharge air sensordevice including means providing for a first relatively large authorityof said discharge air temperature signal when said zone temperature issubstantially at said set point temperature and a second smallerauthority when said zone temperature is substantially different fromsaid set point temperature, said air conditioning system controlledprimarily by said discharge air temperature signal when said authorityis relatively large and controlled primarily by said zone airtemperature signal when said authority is smaller.
 3. A system asclaimed in claim 2 wherein said authority ratio is about 20:1 when theheating or cooling load on said zone is minimal and increases to atleast about 40:1 when the heating or cooling load on said zoneincreases.
 4. A system as claimed in claim 1 wherein said first sensorunit comprises an electrical resistance element having a resistancewhich varies according to the temperature of said element, said elementconnected in sensor circuitry which produces a substantially linearsignal in response to zone air temperature changes within apredetermined range detected at said resistance element.
 5. A system asclaimed in claim 4 wherein said circuit elements of said second sensorunit include a second temperature responsive resistance elementcharacterized by having a resistance which varies in response todischarge air temperature changes, said second resistance elementassociated with circuitry for producing a substantially linear signal inresponse to discharge air temperature changes within a predeterminedrange extending above and below the set point temperature and nonlinearsignals when said discharge air temperatures are beyond said range. 6.In an air-conditioning system for comfort conditioning air circulated toa zone, a control system comprising: a. control means governingoperation of said system in response to sensed air temperatures; b. airtemperature sensor means cooperatively related to said control means; c.said sensor means comprising:
 7. A system as claimed in claim 6 whereinthe ratio of the authority of said zone air temperature signal to saiddischarge air temperature signal is about 20:1 when said zone airtemperature is at said set point temperature, said authority ratioincreasing to at least 40:1 as said zone air temperature varies fromsaid set point temperature.
 8. In a control system for an airconditioned zone: a. air-conditioning apparatus; b. control means forsaid air-conditioning apparatus; c. a zone air temperature sensor; d. adischarge air temperature sensor; e. means for establishing a set pointtemperature; f. one of said sensors including means for changing theauthority thereof as the zone air temperature varies from said set pointtemperature whereby the ratio of the authority of said zone sensor tosaid discharge sensor varies from about 20: 1 when said set pointtemperature and zone temperature are substantially the same to at least40:1 when said zone temperature is substantially different from said setpoint temperature.
 9. A system as claimed in claim 8 wherein tHeauthority of said discharge sensor is reduced as the difference betweensaid zone air temperature and said set point temperature increases.